h a l f b a k e r y
With moderate power, comes moderate responsibility.
add, search, annotate, link, view, overview, recent, by name, random
news, help, about, links, report a problem
or get an account
I want to make a car consisting of a tubular frame with a skin draped over it.
Carbon fiber materials are expensive to make if you want to give them a shape. You have to weave and ply and set this material. There's no real efficient mechanised system to do this.
But you can easily make carbon fiber
tubes, all the same shape. You can produce them en masse.
So you simply take many carbon tubes, attach them to each other via an aluminium piece (I don't know the English word. Is it "joint"?)
Ok, so now you have a very light-weight and strong structure which will protect the passengers.
You put the electric motor, batteries, seats, etc... inside this structure.
Now you can put a skin over this car. A strong, flexible but very thin polymer. (BMW has already such a concept - see link).
This skin is "draped" over lightweight, much thinner tubes, which don't have to be strong because they serve no protection function. They can be made of polyester or another cheap material. These tubes are just needed to give the car an aerodynamic shape, which is obtained by draping the skin over the tubes.
You don't need metal for this skin, as is the case in ordinary heavy cars.
Now you have a much better car than all the other cars. At last we can use carbon fiber in a car, as the main structural element.
I made a rough sketch of it.
As you can see from my exposé, I am an experienced car designer. The idea is not plagiarism from BMW, because I do carbon fiber tubes in an electric car which is not so hot so that the polymer skin can be used in earnest.
Sketch of the car
This is a sketch of the car. [django, Aug 28 2008]
A video about the skin. [django, Aug 28 2008]
Types of bodies and chassis
[mylodon, Aug 28 2008]
Not quite the same idea. [phoenix, Aug 28 2008]
||[+] but why not make the joints out of carbon-fibre also? like plumbing supplies. Note that a skinned car is a return to a frame-based vehicle from the modern "unibody" construction and you may have issues with aerodynamics at high-speed and unattended vandalism.
||/Ok, so now you have a very light-weight and strong structure which will protect the passengers. /
||Two thirds correct. You need some ductility (energy absorption) if you want to protect the passengers.
||Yep, vandalism, didn't think of that. Big problem. But then, the skin would be a rather inexpensive polymer that can be replaced easily (in contrast with a rigid piece of bodywork).
||Carbon fiber joints would be expensive because their form is complex. That's why I thought aluminium.
||// at last we can use carbon fiber in a car // -- see link.
||Use tubes! Of course, why didn't we think of that?
||The problem is the joints, they usually create stress concentrations that make the tubes weak. I'm not saying you can't, as modern CF bike frames have proven, but you seem to be glossing over the tough bit. The interface between the CF and the aluminum can be handled several different ways but since it is nearly impossible to attach to each fiber of a CF tube, the system is a weak point. It's greatest weakness is usually in torsion on the joints, your drawing shows many small tubes which is not the way to go. Gear the design to a few large tubes or pairs of small tubes connected by flat sheets to handle the torsion especially if you are getting rid of the structural skin. I'd also suggest even if you get rid of the classic metal skin, you include other similarly stressed flat skins.
||Also aluminum is also an issue as it stress hardens and becomes brittle. Many racing leagues have banned the use of Al in roll cages for this reason, as they sometimes snap and spear instead of flex and protect.
||FlyingToaster-- It's not really a return to the traditional ladder frame construction, it's more like a spaceframe construction, like the Pontiac Fierro, BMW Z1, or Catterham 7. it's often considered to be better than the more commonly unibody structure. I don't think there would be any high speed aerodyanmics issues if it was properly aerodynamiclly designed, in fact it might be easier to make a very aerodynamic design with this construction. I do see the issues with vandalism, and think the skin might be just not very durable, however it wouldn't be essential to the cars opperation, so it would be totally possible to drive around even if huge holes were ripped in the skin.
||[+] Bun, though there's certainly room for improvement.
||Carbon fiber is only rigid due to the type of resin used
to laminate it and set it into shape.
||Suppose that instead of a stiff resin, a flexible adhesive
was used, and that the tubes were stiffened by inflating
them with compressed air?
||With enough air pressure, the frame could be as rigid as
we need it to be, but since air can always compress
further, it would be flexible enough to absorb energy in
the case of a crash.
||As for the joints... some sort of adapter and joint
system would probably be a good idea.
||For example, create tubular adapters, each several
inches long, which have a gradually varying rigidity
along their lengths. The end of the adapter that's
closes to the joint would be stiff and rigid, the end
that's furthest from the joint would be flexible.
||Glue one adapter inside each end of each piece of
tubing, then fasten the rigid end of each adapter into
||This way, it doesn't matter if the joint itself is overly
rigid, since the adapter spreads out the load on the
||Something similar to this is done when flexible plastic
tubing needs to be attached to brass compression
fittings. A brass or rigid plastic insert is placed inside
the end of the tube before it's attached.
||Lastly, a thin polymer skin sounds excessively
vulnerable to punctures, whether intentional or
accidental. The BMW Gina used a fabric skin... that's
probably the way to go. Either way, the skin ought to
be under high enough tension that it doesn't pull
outward or vibrate when the vehicle moves at high
||//Carbon fiber is only rigid due to the type of resin used to
laminate it and set it into shape. // That's not entirely
true. If the carbon fibre composite is well made, most of
the stiffness comes from the carbon fibre - it has an
excellent specific Young's modulus (stiffness for weight).
Indeed, the resin has to be more flexible than the carbon
fibres, or else the load winds up being taken by the resin
which will crack. The resin is there to hold the fibres
together into the right shape.
||//With enough air pressure, the frame could be as rigid as
we need it to be// Definitely not true for tubes which are
relatively long compared to their diameter. Whatever the
wall thickness of your tubes, if you pressurize them to
bursting point then only an insignificant amount of the
resulting stiffness is due to the air pressure. Basically,
bending a long tube has only a very very small effect on its
volume, which is another way of saying that pressurising it
will have a negligible effect on its stiffness.
||//since air can always compress further, it would be flexible
enough to absorb energy in the case of a crash.//
Definitely not true, but this time for several reasons. First,
the tube will break before the air pressure inside has been
raised by even a small amount - carbon fibre composites are
generally quite brittle, which is the price paid for the
stiffness. Second, absorbing kinetic energy in a
compressed gas is a very very bad idea. You might as well
put a couple of big springs on the front of the car. To
mitigate the effects of a crash, you have to absorb the
energy non-reversibly. That is why steel car bodies are
about the safest boxes around - deforming all that metal
absorbs a lot of energy and keeps it there.
||This seems like the century-old method of
covering wood or metal framework with doped
fabric, as used in aircraft construction, except
that composite framework material appears to be
used instead of wood or metal. Since the body
covering doesn't appear to need to be load-
bearing, a cheap fabric like rayon or dacron could
be used, painted with dope, which would tighten
the skin over the framework as the dope dries.